As it is generally known, labelling machines are used to apply labels to containers of all sort. Typically used with beverage bottles or vessels are tubular labels (commonly called “sleeve labels”), which are obtained by:                cutting the web unwound from a supply roll into a plurality of rectangular or square labels;        bending each label in a tubular configuration such that the opposite vertical edges overlap one another; and        welding the overlapped edges of each tubular label.        
A particular type of labelling machine is known, in which each tubular label is formed onto a relative cylindrical winding body (commonly called “sleeve drum”) in order to be then transferred onto the relative container, for instance by inserting the latter inside the tubular label.
This type of labelling machine basically comprises a carousel rotating about a vertical axis to define a circular path, along which it receives a succession of unlabelled containers and, then, a succession of rectangular or square labels from respective input wheels, allows application of the labels in a tubular configuration onto the respective containers and releases the labelled containers to an output wheel.
More specifically, the carousel comprises a number of operating units which are equally spaced about the rotation axis, are mounted along a peripheral edge of the carousel and are moved by the latter along the above-mentioned circular path.
Each labelling unit comprises a bottom supporting assembly adapted to support the bottom wall of a relative container and an upper retainer adapted to cooperate with the top portion of such container to hold it in a vertical position during the rotation of the carousel about the vertical axis.
Each supporting assembly comprises a vertical hollow supporting mount, secured to a horizontal plane of rotary frame of the carousel, and a cylindrical winding body, engaging the supporting mount in sliding and rotating manner with respect to its axis, and adapted to carry a relative container on its top surface and a relative label on its lateral surface.
Each winding body is movable, under the control of cam means, between a raised position and a fully retracted position within the relative supporting mount.
In the raised position, each winding body protrudes from a top surface of the relative supporting mount and is adapted to receive a relative label on its lateral surface from the label input wheel; in particular the label is wrapped around the winding body such that the opposite vertical edges of the label overlap one another.
In the fully retracted position, which is reached at the container input and output wheels, the top surface of each winding body is flush with the top surface of the supporting mount so that containers are transferred onto and from the carousel along the same transfer plane.
After the welding of the overlapped edges of a tubular label, the movement of the relative winding body from the raised position to the fully retracted position produces the insertion of the relative container inside the label, making the so obtained container ready to be transferred to the output wheel.
Recently, it has been proposed to use the ultrasound technology for welding the overlapped edges of the tubular labels.
An example of the application of this technology to the labelling field is shown in the International Patent Application No. WO 2005/085073.
According to this solution, each ultrasound welding device is arranged on the carousel in a radially inner position with respect to the relative winding body.
More specifically, each ultrasound welding device is assembled on a bracket of a relative turret carried by the carousel in front of a relative labelling unit.
In use, each welding device is first moved horizontally in a radial direction with respect to the carousel rotation axis towards the label overlapped edges to be welded and, then, once having come in contact with the latter, is moved vertically for the whole height of the overlapped edges to perform the welding action.
After having ended the welding step, each welding device is detached from the relative label and returned to its starting position along an analogous path, including a first horizontal movement and a second vertical movement.
The above-described solution has the following drawbacks.
First of all, the ultrasound technology does not permit to weld any type of labels; for instance, some particular colors may render the labels opaque to ultrasounds.
Besides, to obtain welding of the label overlapped edges through the ultrasound technology is necessary to operate at high frequencies which may produce some inaccuracies in the welding operation.
Moreover, possible presence of micro-fractures in the sonotrode of an ultrasound welding device may produce stop of the ultrasound waves flow at certain frequencies; with consequent unsuccessful welding.
Furthermore, due to the fact that the ultrasound welding operation is carried out by a sequence of four movements according to two Cartesian axes, the whole time for completing this operation is in general relatively long. By also considering that one ultrasound welding device is provided for each winding body, the system for controlling and performing the welding operations on a labelling machine of this type is extremely complex.